The present invention relates to a magnetic resonance (MR) image production method and a magnetic resonance imaging (MRI) system. More particularly, the present invention relates to an MR image production method and an MRI system capable of accurately producing separate water and fat images using a plurality of coils despite a small number of arithmetic operations and a short processing time.
In the past, a technique of producing separate water and fat images such as a linear combination (LC) steady-state free precession (SSFP) technique, a Dixon technique, or a three-point Dixon technique, that is, a technology for producing a water image or a fat image by utilizing a phase difference between a water signal and a fat signal has been known (refer to, for example, Patent Document 1, Patent Document 2, and Non-patent Document 1).
On the other hand, a parallel imaging technology falling into an image synthesis method such as a sum-of-square method and a sensitivity encoding (SENSE) method, that is, a technology for receiving nuclear magnetic resonance (NMR) signals, which are induced by a subject, in parallel with one another using a plurality of coils, and processing them to produce one MR image has been known (refer to, for example, Patent Document 3 and Non-patent Document 2).
[Patent Document 1] Japanese Patent No. 3353826
[Patent Document 2] Japanese Unexamined Patent Application Publication No. 2003-52667
[Patent Document 3] Japanese Unexamined Patent Application Publication No. 2003-79595
[Non-Patent Document 1] “Linear Combination Steady State Free Precession MRI” (Vasanawala et al., Magnetic Resonance in Medicine, Vol. 43, 2000, pp. 82–90)
[Non-Patent Document 2] “SENSE: Sensitivity Encoding for Fast MRI” (Klaas P. Pruessmann et al., Magnetic Resonance in Medicine, Vol. 42, 1999, pp. 952–962)
A conventional method of producing separate water and fat images using a plurality of coils comprises, for example, steps (1) to (4) described below.
(1) A pulse sequence that causes a fat signal to have a phase difference of −90° from a water signal is applied in order to receive NMR signals, which are induced by a subject, in parallel with one another using I (I≧2) coils, whereby complex images H−R(1) to D−R(I) are produced in association with the coils.
(2) A pulse sequence that causes a fat signal to have a phase difference of +90° from a water signal is applied in order to receive NMR signals, which are induced by a subject, in parallel with one another using the I coils, whereby complex images H+R(1) to H+R(I) are produced in association with the coils.
(3) Water images w(i)=H−R(i)+H+R(i) are produced in association with the coils, and fat images f(i)=H−R(i)−H+R(i) are produced in association therewith (where i denotes, 1, 2, etc., or I).
(4) The sum-of-square method based on the parallel imaging technology is adopted in order to synthesize the water images w(i) associated with the coils, whereby one water image W is produced. Likewise, the fat images f(i) associated with the coils are synthesized in order to produce one fat image F.
However, according to the above method, at step (3), arithmetic operations must be repeated by the same number of times as the number of coils. This poses a problem in that the total number of arithmetic operations increases and a processing time extends.
Moreover, any other conventional method for producing separate water and fat images using a plurality of coils, and any other conventional parallel imaging technology may be used in combination. However, the conventional parallel imaging technology employs a gradient echo production pulse sequence for the purpose of shortening a scan time. The gradient echo production pulse sequence brings about a phase difference between a water signal and a fat signal from which calibration data is detected. Therefore, a phase difference between a water signal and a fat signal from which real data is detected is lost during synthesis. This poses a problem in that separate water and fat images cannot be produced accurately.